The present invention relates to a method of forming a regular network of semiconductor islands on an insulating substrate. Such semiconductor islands can be used for manufacturing quantal devices such as devices using a Coulomb blocking phenomenon.
The Coulomb blocking phenomenon occurs in conductive or semiconductor islands both insulated electrically from their environment and weakly coupled thereto by tunnel effect. Using this phenomenon at temperatures close to room temperatures requires the total capacitance of each island to be around 1 attofarad. The dimensions of the islands are generally around 1 nanometer.
The invention finds applications notably in the manufacture of logic circuits and memories with a very high integration density.
The accompanying FIG. 1 depicts a highly schematic plan view of a device 10 using semiconductor islands.
References 12 and 14 designate first and second electron reservoirs of the quantal effect device 10, whose functioning uses the Coulomb blocking phenomenon. These reservoirs are, for example, the drain and source of a structure of the field effect transistor type, or a microelectronic device such as a memory.
Between the two electron reservoirs 12, 14 there is a region with a set of semiconductor islands 16, or grains.
During the manufacture of such a region, it is found that the formation of the islands is random and irregular.
When the formation of the islands is obtained by a nucleation process, it complies with a statistical distribution law for the nucleation centres fixed by known thermodynamic laws and set out, for example, in the article xe2x80x9cThe Nucleation of CVD Silicon on SiO2 and Si3N4 Substratesxe2x80x9d by W. Claassen, et al., Journal of the Electrochemical Society 128, No 6, pp. 1353-1359, (1981). It is known for example that a silicon nitride surface is more favorable to a high nucleation density than a silica (SiO2) surface since the mechanism for the deposition of silicon from silane is based on the formation of species of the SiH2 type which diffuse rapidly on a surface with a high density of OH bonds, such as an SiO2 surface.
It is also known that the nucleation density of silicon can be increased by specific treatments. In a treatment described in the article xe2x80x9cSurface Treatment Effect on the Grain Size and Surface Roughness of as-Deposited LPCVD Polysilicon Filmsxe2x80x9d by A. T. Voutsas, et al., Journal of the Electrochemical Society 140, No 1, pp. 282-288, (1993), the nucleation densities obtained after cleaning of the silica in a chemical bath (notably in a bath based on sulphuric acid and hydrogen peroxide), are greater than those obtained without treatment. This phenomenon is probably attributable to the presence of impurities left by the bath on the surface of the silica.
However, the nucleation, even when it is assisted by impurities, remains a statistical phenomenon, which does not make it possible to create regularly spaced islands of silicon.
The irregularity of the distribution of the nucleation kernels, and therefore of the islands of semiconductor material, is accompanied by a lack of homogeneity of the size of the islands. This phenomenon limits the quality and performance of the Coulomb blocking electronic devices, using such a structure.
The purpose of the present invention is to propose a method of forming a network of islands regularly spaced apart on an electrically insulating support.
One aim is also to propose such a method for obtaining islands of homogeneous size.
Another aim is to propose an electronic device of the Coulomb blocking type using a network of regularly spaced islands, obtained in accordance with the invention.
To achieve these aims, the object of the invention is more precisely a method of forming a network of islands of semiconductor material on a surface of an electrically insulating material. The method comprises:
a) the deposition of nucleation kernels on the surface of the electrically insulating material, by means of an auto-organisation process,
b) the formation of islands of semiconductor material on each of the nucleation kernels.
In accordance with the invention, the deposition of the nucleation kernels is effected using at least one layer, referred to as a distribution layer, made of a material having a substantially regular molecular structure, formed on the electrically insulating material surface, in order to distribute the nucleation kernels in a substantially regular fashion on the surface of the electrically insulating material by means of an auto-organisation process.
Semiconductor islands means grains of semiconductor material formed by growth on the nucleation kernels. These grains have small dimensions of between, for example, 1 and 10 nanometers. Their growth can be assisted or promoted by means of a heat treatment.
In addition, nucleation kernel means any impurity able to promote the local formation of a semiconductor grain, in particular a crystal. The impurity can cause the formation of the crystal either directly, or indirectly by causing in the surface of the electrically insulating material a local structure modification able to promote the formation of the semiconductor crystal. The kernels are, in particular, atoms.
The distribution layer can, in accordance with particular embodiments of the invention, be used either as a mask for the deposition of kernels, or directly as a kernel addition layer.
According to the first particular embodiment proposed, use is made of a distribution layer made of a material with a molecular structure having interstices spaced apart substantially regularly, and the nucleation kernels are deposited using the distribution layer as a deposition mask for uniformly distributing the nucleation kernels, the distribution layer being eliminated after the deposition of the nucleation kernels.
By way of example, the material used for the distribution layer can be an organic material of the phthalocyanine or porphyrin type (a molecule with a porphyry core). These molecules can be functionalised, that is to say can carry substituents, so as to initiate between them chemical bonds with fixed lengths. This auto-organisation process makes it possible to bring the kernels (in particular metallic centres) to a fixed and determined distance from each other.
The kernels are, for example, made of a metal such as, preferentially, Al, Mg, Ca, Se. These metals, in atomic form, are able to interact on the surface of the silica. Other metals such as Cu or Ni can also be used. These metals are however liable to diffuse, even at low temperature, in the insulating material, when the latter is made of silica.
The kernels are distributed on the distribution layer in order to fit in the interstices and be fixed on the insulating surface in the interstices. The fixing of the kernels on the insulating surface takes place notably by chemical sorption.
To allow an easy fitting of the kernels in the interstices of the distribution layer, this layer can, preferably, be produced in a monolayer form, that is to say in the form of a monomolecular layer. In this case, the molecules are functionalised so as to make them suitable for deposition in a monomolecular layer.
The formation of a monomolecular layer can take place, for example, according to a technique, known per se, referred to as the Langmuir-Blodgett technique. In this regard reference can be made to the French document FR-A-2 666 092 (Feb. 28, 1992.).
According to the second particular embodiment of the proposed invention, use is made of a distribution layer made of a material having a molecular support structure and nucleation kernels regularly distributed on the molecular support structure. After putting this layer on the surface of the insulating material, a treatment is carried out to separate the nucleation kernels, fixed on the surface of the insulating material, and the support structure, in order to eliminate this structure.
The material of the distribution layer is for example an organometallic material having metal sites, forming the kernels, in a molecular structure having auto-organisation properties in order to ensure a regular distribution of the metallic sites on the surface of the insulating material. By way of example, the material of the distribution layer can consist of porphyrins or phthalocyanines, or cage molecules complexing metallic ions, such as calixarenes and cyclodextrins.
By way of example, when the insulating material is silica (SiO2), the nucleation kernels can include a metal chosen from amongst Fe, Al, Ca and Mg. The deposition of the nucleation kernels is accompanied by a heat treatment enabling the metal to interact with the silica.
A heat treatment then allows modification of the silica surface to locally form compounds of the silicate type such as FeSiO4, xc2xdMg2SiO3, xc2xdMg2SiO4, or CaSiO4 or of the tetrahedral aluminum type. In his regard reference can be made to documents xe2x80x9cDielectric Degradation of Silicon Dioxide Films Caused By Metal Contaminationsxe2x80x9d by M. Takiyama, et al., 7th International Symposium on Silicon Material Science and Technology PV 94-10 (Electrochemical Society 1994) and xe2x80x9cThe Colloid Chemistry of Silica and Silicatesxe2x80x9d, by R. K. Iler, p. 250 Cornell University Press (1955).
These compounds locally modify the surface of the silica and thus promote the formation of semiconductor islands.
The treatment, in order to separate the nucleation kernels and the molecular support structure of the distribution layer, includes a heat treatment and/or a treatment by ultraviolet radiation. This treatment destroys and eliminates the molecular support structure.
Another object of the invention is a method of manufacturing a quantal device of the Coulomb blocking type having a network of semiconductor islands, in which the network of islands is produced in accordance with the method described above.
Finally, the invention relates to an electronic device of the Coulomb blocking type, comprising first and second electron reservoirs separated by a region including a set of nanometric semiconductor islands formed on an electrically insulating substrate, and in which the islands are regularly spaced apart on the electrically insulating substrate.
The first and second reservoirs are, for example, the source and drain of a transistor structure or memory cell structure.
Other characteristics and advantages of the present invention will emerge more clearly from the following description, with reference to the figures of the accompanying drawings. This description is given purely for illustration and non-limitatively.